The vibrating probe detects small currents around organs and organisms. There are outward currents at the equator of the lens and inward currents at the optical poles. The former appear to be K+ currents and the latter Na+ currents. The system has been expanded by adding microelectrodes so that in addition to measuring currents, J, it is also possible to measure the potential difference across lens membranes, PD, and to determine the effect of injecting current, l, on the PD and on J. Quabain has little immediate effect of injecting current, I, on the PD and on J. Quabain has little immediate effect on the currents, J. The outward and inward currents are driven by their electrochemical gradients and are segments of ionic current loops that describe the flow of the ions through and around the lens. The segment that is studied consists of the ion pathway starting at the tip of the PD measuring electrode, passing through the cytosol and gap junctions, then through the lens membranes into the extracellular fluid and thus reaching and ending at the vibrating probe. The resistance of the membranes is quantal times greater than that of the cytosol or extracellular fluid. Therefore, the major factor affecting the current, J, is the resistance of the membranes. This in turn is determined by the ionic channels in the membrane. Thus, the system consisting of a vibrating probe combined with microelectrodes provides a unique method for studying, at the level of the whole lens, the effects of perturbations on ionic channels. The system permits the determination of the parameters of Ohm's law so that currents can be described on a quantitative basis. The equatorial current is a relatively pure K+ current that can be related to the 45 pS K+ channel that has been found in lens membranes. The study of this current is being continued. The inward current probably relates to the dominant 25 pS non-selective cation channel of the lens. This is a Na+ conducting channel and remains to be studied. The non-selective cation channel is "stretch activated" so that the resulting currents may relate to the process of accommodation.